High pressure, high temperature gasification systems have been used to partially oxidize hydrocarbonaceous fuels to recover useful by-products or energy. The fuels can be admixed with water to form an aqueous feedstock that is fed to the reaction zone of a partial oxidation gasifier along with an oxygen containing gas and a temperature moderator.
Mixing the feed with water may not be necessary, given the composition and physical nature of the feedstock. Generally, solid carbonaceous fuels will need to be liquefied with oil or water prior to feeding to the gasifier. Liquid and gaseous hydrocarbonaceous fuels may be suitable for direct feed to the gasifier, but can be pre-treated for removal of any impurities that might be present in the feed.
The term liquid hydrocarbonaceous fuel as used herein to describe various suitable feedstocks is intended to include pumpable liquid hydrocarbon materials and pumpable liquid slurries of solid carbonaceous materials, and mixtures thereof. In fact, any combustible carbon-containing liquid organic material, or slurries thereof may be included within the definition of the term xe2x80x9cliquid hydrocarbonaceous.xe2x80x9d For example, there are pumpable slurries of solid carbonaceous fuels, liquid hydrocarbon fuel feedstocks, oxygenated hydrocarbonaceous organic materials, and mixtures thereof. Gaseous hydrocarbonaceous fuels may also be processed in the partial oxidation gasifier alone or along with liquid hydrocarbonaceous fuel.
The partial oxidation reaction is preferably carried out in a free-flow, unpacked non-catalytic gas generator, or gasifier at a temperature within the range of about 700xc2x0 C. to about 2000xc2x0 C., preferably about 1200xc2x0 C. to about 1500xc2x0 C. The gasifier operates at a pressure of about 2 to about 250 atmospheres, preferably about 10 to about 150 atmospheres, and most preferably about 20 to about 90 atmospheres. Under these conditions, about 95% to 99.99% of the hydrocarbonaceous feedstock can be converted to a synthesis gas containing carbon monoxide and hydrogen, also referred to as synthesis gas or syngas. Carbon dioxide and water are either formed or consumed via water gas shift reaction [CO+H2OCO2+H2] depending on the type of the moderator employed and operating conditions.
Water is further used as quench water to quench and cool the syngas. In a typical gasification reactor, the effluent gas passes out the bottom of the gasification reactor into a quench chamber. The effluent gas is cooled by passing through a pool of quench water. The quench water cools the syngas and scrubs particulate matter from the syngas, and is further used to convey particulate waste solids, such as ash and/or slag out of the gasifier. Generally, ash and/or slag is allowed to accumulate in the bottom of the quench chamber, and periodically that ash and/or slag is removed from the quench chamber using a lockhopper system. The syngas leaves the quench chamber through an outlet port above the level of the quench water. Quench water is continuously circulated in the quench chamber, being removed from the quench chamber as soot water, carbon water, or black water at an outlet port below the level of the quench water.
Removal of particulate carbon from the soot water is commonly done in a soot recovery unit. One common process is solvent extraction in a one or two-stage decanter, such as shown and described in U.S. Pat. No. 4,014,786, or in a traditional naphtha extraction unit. These carbon extraction systems are complex and have high capital costs due to the large number of equipment items, cost of solvent, and high energy costs. The carbon can also be removed from the soot water by using a conventional liquid-solids separator such as a filter press, hydrocyclone, or centrifuge. For example, by means of a filter press, filter cake having a solids content of 10 to 60 wt. % may be produced along with a grey water filtrate. The filter cake is usually disposed of, and preferably a portion of the grey water is recycled back to the gasification reactor for use as quench water. Such a process employing a filter press is shown and described in U.S. Pat. No. 5,415,673.
Even after having the solids removed, the composition of the grey water is fairly complex. This water can contain chlorides, ammonium salts, and other environmentally harmful dissolved materials such as sulfide and cyanide. Heavy metals such as antimony, cadmium, chromium, cobalt, lead, molybdenum, nickel, strontium and zinc, are also remain in the grey water.
Because of this, after soot removal, the entire grey water stream cannot be recycled back to the gasification system because not all the impurities are removed in the soot recovery unit. Instead, a significant portion of the grey water must be treated in a wastewater treatment unit to remove these impurities from the grey water, so that the concentrations of these impurities do not build up in the recycled grey water stream. Some compounds found in the water, particularly the heavy metals, are very difficult to remove from the grey water. It is costly to treat large amounts of grey water with high metals concentrations. Thus, it would be desirable to develop a process that would minimize the metals content in the grey water so that more grey water could be recycled back to the gasification system, reducing the size of the wastewater treatment as it would have to handle less grey water.
In accordance with the present invention, a basic material, preferably ammonia, is injected into the stream of soot water leaving the gasification reactor prior to being processed in a filtration-type soot removal unit. The elevated pH of the soot water causes more metals to remain in the filter cake. Thus, the metals content of the grey water is reduced. More grey water can then be recycled back to the gasification unit, with less grey water being handled by the wastewater treatment unit. This advantageously increases the efficiency of the overall gasification process, and reduces the size and process costs of the wastewater treatment unit because there are less metals to be removed from the grey water.